1. Field of the Invention
This invention relates to an automatic speed control system for a vehicle and more specifically, to a speed control for an uphill road use when the vehicle runs on an uphill running road.
2. Description of Related Art
There has been known an automatic speed control system for a vehicle in which a throttle valve opening is controlled based on a deviation between a target vehicle speed and an actual vehicle speed so that the vehicle speed is controlled to a predetermined value such a system is disclosed in Japanese Patent Publication 59-192114, laid open to the public in 1984.
Generally, in such a conventional speed control system, a shift stage change timing is changed based on the deviation between the target vehicle speed and the actual vehicle speed, the set time of a shift operation prohibit timer and the like when the vehicle runs in an uphill and downhill path. The vehicle speed is controlled to a target value in an uphill running condition and a downhill running condition, as disclosed in Japanese Patent Publication No. 62-36889, published for opposition in 1987, and Japanese Patent Public Disclosure No. 63-137037, laid open to the public in 1988.
For instance, in a conventional speed control system, when the vehicle is about to run into an uphill path, a down-shift operation is made in automatic transmission of the vehicle to provide enough a power to run the uphill path. Thereafter, a up-shift operation is made when a predetermined time has passed so that a proper shift stage can be obtained in response to the actual vehicle speed.
It should, however, be noted that such a proposed automatic speed control system has some disadvantages.
Since the timing of the up-shift operation is predetermined in accordance with the length of the uphill path, the up-shift operation may not be timely provided when the length of the uphill path is not consistent with the expected value. Such may occur, for example, when the path length is longer than the expected value or shorter than the expected value. As a result, in some cases, the down shift condition may be undesirably maintained even after the uphill path has passed. This causes an increased noise level, due to the fact that the vehicle runs in too low a gear stage. Alternatively, the up-shift condition may be established while the vehicle is still on the uphill path when the length of the path is longer than the expected value. In this case, the down-shift operation may be repeated to compensate for the speed drop of the vehicle. This may cause what is known as "hunting" in the speed control.